Accurately quantifying food intake is vital to promoting health and reducing chronic disease risk. Food intake encompasses energy intake, nutrient intake, and intake of various food groups (e.g., fruits, vegetables), and thus reflects the nutritional status of individuals. Nutrition affects disease risk, including risk of developing obesity, diabetes, and cancer, all of which negatively affect the United States (U.S). Despite its importance, accurately quantifying food intake has challenged researchers and clinicians for decades. Self-report methods (e.g., food records and diet recall) are a mainstay of nutritional epidemiology research, but their accuracy has been questioned, due, in part, to missing data and people inaccurately estimating portion size and recalling what they ate. Advances in assessing food intake over the past 15 years include technology-assisted approaches, including those that rely on food photography. Our group previously developed the Remote Food Photography Method (RFPM) and SmartIntake app, which quantifies food intake based on food images that users capture before and after they eat. Accurate estimates of food intake are obtained with this method in most study populations and settings, yet analysis of the images takes time and resources, requires a human rater, and users do not receive immediate feedback about their food intake. We developed the PortionSize smartphone app to overcome these limitations. The PortionSize app relies on users capturing images of their food selection and waste, but it immediately provides users with food intake data. The PortionSize app includes innovative technology to minimize missing data and to help users accurately estimate portion size. Preliminary data supports the validity of the PortionSize app, and during the proposed research the reliability and validity of PortionSize and MyFitnessPal, a commonly used smartphone-based food record, will be tested against `gold-standard' criterion measures. Specifically, the apps will be tested in healthy adults under the following three conditions: 1) laboratory-based test meals (Study 1), 2) free-living conditions, where participants will consume pre-weighed food from a cooler, which provides a test of energy and nutrient intake in free-living conditions (Study 2), and 3) free-living conditions, where energy intake is also assessed by doubly labeled water (Study 3). If found to be valid, the PortionSize app will move the field forward by providing a method that could widely and affordably be disseminated to assess food intake and foster/track adherence to personalized diets in real time.